Stewart’s former lover, Colin Howell, is currently serving a 21-year sentence after pleading guilty to gassing their spouses as they slept.

For almost two decades, police in Northern Ireland were convinced that the pair had taken their own lives in a bizarre suicide pact until Howell confessed to elders in his church group two years ago.

Stewart’s trial lasted three and a half weeks.

Outside the court in Coleraine the victims relatives expressed their relief that justice had finally being served.

The policeman’s brother Gordan told reporters that the revelations over how he died extended the families’ grief.

"While there is immense satisfaction that justice for Trevor has finally been achieved, there is no sense of victory and no cause for celebration as nothing can bring Trevor and Lesley back to us, and all families connected to this matter have been grievously impacted," he said.

Lauren Bradford, Howell’s daughter, paid tribute to her mother.

"We mourn our mother, Lesley, and are pained at the time and the memories that we have been so denied," she said as relations, including Howell's brother, Christopher Clarke, stood beside her.

"We rejoice in the contribution our mum made to our lives in the short time we had together. We know her to have been a loving, devoted mother, and we bitterly regret the horrible way in which she was taken from us."

Buchanan, 32, and Howell, 31, were found dead in a car filled with automobile fuels in a garage in Co. Derry.

Investigators believed they had died in some sort of suicide pact due to the shared distress of their spouses’ affairs.

Howell and Stewart, who split up five years after they carried out the murders, kept the murders under wraps for almost 20 years.

A former dentist, Howell gassed his wife as she slept on the sofa in their home in Coleraine. He then drove to the other side of the town where he murdered Buchanan in his house by the same method before taking the two bodies away to stage the “suicide.”